The skin is made up of several layers with the upper composite layer being the epithelial layer. The outermost layer of the skin is the stratum corneum which has well known barrier properties to prevent molecules and various substances from entering the body and analytes from exiting the body. The stratum corneum is a complex structure of compacted keratinized cell remnants having a thickness of about 10-30 microns. The stratum corneum forms a waterproof membrane to protect the body from invasion by various substances and the outward migration of various compounds.
The natural impermeability of the stratum corneum prevents the administration of most pharmaceutical agents and other substances through the skin. Numerous methods and devices have been proposed to enhance the permeability of the skin and to increase the diffusion of various drugs through the skin so that the drugs can be utilized by the body. Typically, the delivery of drugs through the skin is enhanced by either increasing the permeability of the skin or increasing the force or energy used to direct the drug through the skin. Examples include the use of micro-needles, laser ablation, RF ablation, heat ablation, sonophoresis, iontophoresis, or a combination thereof.
In some cases, these treatments are provided by a single integrated device that performs two separate functions: the device treats the skin and also delivers an active ingredient to the treated skin. An example is a hollow micro-needle array with an attached drug reservoir.
Transdermal patches are well known and in wide use for many purposes, such as but not limited to, hormonal therapy, narcotic analgesia, nicotine and many others. However, transdermal patches have drawbacks and patch administration can be complicated. For example, many patients fail to remove the overlay or protective liner in order to expose the adhesive and medication to the skin for absorption. Of course, the result is the medicine is not administered to the patient. Another problem is overdose; applying too many patches at one time can be fatal (e.g., fentanyl overdose). Another problem is changing the patch for a fresh patch at the wrong time interval, thereby causing improper dosage of the medicine. Accordingly, patient compliance in using transdermal patches is a critical problem facing caregivers (especially for elderly patients) and pharmaceutical firms alike.
In summary, it is often difficult to achieve proper time duration of patch application, proper intervals between dosages and proper dosing strengths.